1. Field of the Invention
The invention is related to magnetic resonance imaging methods of the type that employ RF transmission to excite nuclear spins only inside a portion of the object.
2. Description of the Prior Art
In magnetic resonance imaging (MRI), a radio-frequency (RF) signal is transmitted to the sample (examination subject) in order to excite proton spins inside the sample, in order to generate an MR signal. To encode this signal spatially, a group of gradient fields are used. Conventionally, these gradient fields have linear dependence on the spatial coordinates, x, y and z. Hence, their effects on the spins are linear, and the governing equations are Fourier transforms in MRI.
Depending on the time of application during a pulse sequence, the encoding schemes accomplished by the gradient fields are called slice selection, phase encoding and readout (or frequency encoding). It is known that for linear gradient fields, the slice profile (or, excitation profile) along the slice selection direction, is the Fourier transform of the RF pulse (Pauly J, Nishimura D, Macovski A, A k-Space Analysis of Small-Tip-Angle Excitation. J Magn Reson, 1989; 81:43:56). In most MRI experiments, it is desirable to excite the spins inside the sample so that they have a perfectly rectangular slice profile, i.e., exciting the spins only between two planes while not exciting the other spins. Therefore, because of the Fourier transform relation, most sequences use sinc-type RF pulse envelopes.
The specific absorption rate (SAR), which is one of the most important parameters of an MRI experiment, is the total absorbed power inside a sample, per certain mass of tissue. Regulations are given by the FDA and IRC for 1-gram, 10-gram, head, whole body and organ averaged (torso and extremities) power values. The total absorbed power inside a sample is proportional to the magnitude squared integral of the RF envelope. Therefore, sinc-type envelopes have higher SAR than smoother envelopes.
It has been shown that, without altering the slice profile, SAR of an RF envelope can be decreased using variable-rate selective excitation (VERSE), (Conolly S, Nishimura D, Macovski A, Variable-Rate Selective Excitation. J Magn Reson, 1988; 78:440-458). In conventional techniques, the gradient coils that produce the slice selection gradients are turned on (activated) during slice selection and turned off afterwards. However, with VERSE technique, the current waveform that feeds the gradient coils is made a function of time. This way, the RF envelope can be modified without changing the slice profile. However, because the gradient currents vary in time, the off-resonance effects become a function of time and cause blurring.
An alternative way to alter the relation between the slice profile and the RF envelope is to use nonlinear gradient fields. Although nonlinear gradient fields have been studied before (for example, Lee S Y, Cho Z H, Localized Volume Selection Technique Using an Additional Radial Gradient Coil. Magn Reson Med, 1989; 12:56-63), their effect on the slice profile has not been recognized. Hence, the usage of nonlinear gradients has remained to serve the purpose of phase and frequency encoding.